The present invention relates to an artificial implant to be placed into the intervertebral space left after the removal of a damaged spinal disc. Specifically, the invention concerns an implant that facilitates arthrodesis or fusion between adjacent vertebrae while also maintaining or restoring the normal spinal anatomy at the particular vertebral level.
The number of spinal surgeries to correct the causes of low back pain has steadily increased over the last several years. Most often, low back pain originates from damage or defects in the spinal disc between adjacent vertebrae. The disc can be herniated or can be suffering from a variety of degenerative conditions, so that in either case the anatomical function of the spinal disc is disrupted. The most prevalent surgical treatment for these types of conditions has been to fuse the two vertebrae surrounding the affected disc. In most cases, the entire disc will be removed, except for the annulus, by way of a discectomy procedure. Since the damaged disc material has been removed, something must be positioned within the intradiscal space, otherwise the space may collapse resulting in damage to the nerves extending along the spinal column.
In order to prevent this disc space collapse and to stabilize the spine, the intradiscal space is filled with bone or a bone substitute in order to fuse the two adjacent vertebrae together. In early techniques, bone material was simply disposed between the adjacent vertebrae, typically at the posterior aspect of the vertebrae, and the spinal column was stabilized by way of a plate or a rod spanning the affected vertebrae. With this technique once fusion occurred the hardware used to maintain the stability of the segment became superfluous. Moreover, the surgical procedures necessary to implant a rod or plate to stabilize the level during fusion were frequently lengthy and involved.
It was therefore determined that a more optimum solution to the stabilization of an excised disc space is to fuse the vertebrae between their respective end plates, most optimally without the need for anterior or posterior plating. There have been an extensive number of attempts to develop an acceptable intradiscal implant that could be used to replace a damaged disc and yet maintain the stability of the disc interspace between the adjacent vertebrae, at least until complete arthrodesis is achieved. These xe2x80x9cinterbody fusion devicesxe2x80x9d have taken many forms. For example, one of the more prevalent designs takes the form of a cylindrical implant. These types of implants are represented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In these cylindrical implants, the exterior portion of the cylinder can be threaded to facilitate insertion of the interbody fusion device, as represented by the Ray, Brantigan and Michelson patents. In the alternative, some of the fusion implants are designed to be pounded into the intradiscal space and the vertebral end plates. These types of devices are represented by the patents to Brantigan, U.S. Pat. Nos. 4,743,256; 4,834,757 and 5,192,327.
In each of the above listed patents, the transverse cross section of the implant is constant throughout its length and is typically in the form of a right circular cylinder. Other implants have been developed for interbody fusion that do not have a constant cross section. For instance, the patent to McKenna, U.S. Pat. No. 4,714,469 shows a hemispherical implant with elongated protuberances that project into the vertebral end plate. The patent to Kuntz, U.S. Pat. No. 4,714,469, shows a bullet shaped prosthesis configured to optimize a friction fit between the prosthesis and the adjacent vertebral bodies. Finally, the implant of Bagby, U.S. Pat. No. 4,936,848 is in the form of a sphere which is preferably positioned between the centrums of the adjacent vertebrae.
Interbody fusion devices can be generally divided into two basic categories, namely solid implants and implants that are designed to permit bone ingrowth. Solid implants are represented by U.S. Pat. Nos. 4,878,915; 4,743,256; 4,349,921 and 4,714,469. The remaining patents discussed above include some aspect that permits bone to grow across the implant. It has been found that devices that promote natural bone ingrowth achieve a more rapid and stable arthrodesis. The device depicted in the Michelson patent is representative of this type of hollow implant which is typically filled with autologous bone prior to insertion into the intradiscal space. This implant includes a plurality of circular apertures which communicate with the hollow interior of the implant, thereby providing a path for tissue growth between the vertebral end plates and the bone or bone substitute within the implant. In preparing the intradiscal space, the adjacent end plates are preferably reduced to bleeding bone to facilitate this tissue ingrowth. During fusion, the metal structure provided by the Michelson implant helps maintain the patency and stability of the motion segment to be fused. In addition, once arthrodesis occurs, the implant itself serves as a sort of anchor or scaffold for the solid bony mass.
A number of difficulties still remain with the many interbody fusion devices currently available. While it is recognized that hollow implants that permit bone ingrowth into bone or bone substitute within the implant are an optimum technique for achieving fusion, most of the prior art devices have difficulty in achieving this fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of these devices are not structurally strong enough to support the heavy loads and bending moments applied at the most frequently fused vertebral levels, namely those in the lower lumbar spine.
There has been a need for providing a hollow interbody fusion device that optimizes the bone ingrowth capabilities but is still strong enough to support the spine segment until arthrodesis occurs. It has been found by the present inventors that openings for bone ingrowth play an important role in avoiding stress shielding of the autologous bone impacted within the implant. In other words, if the ingrowth openings are improperly sized or configured, the autologous bone will not endure the loading that is typically found to be necessary to ensure rapid and complete fusion. In this instance, the bone impacted within the implant may resorb or evolve into simply fibrous tissue, rather than a bony fusion mass, which leads to a generally unstable construction. On the other hand, the bone ingrowth openings must not be so extensive that the cage provides insufficient support area to avoid subsidence into the adjacent vertebrae.
The use of bone graft materials in past metal cage fusion devices has presented several disadvantages. Autograft is undesirable because existing structures may not yield a sufficient quantity of graft material. The additional surgery to extract the autograft also increases the risk of infection and may reduce structural integrity at the donor site. Furthermore, many patients complain of significant pain for several years after the donor surgery. Although, the supply of allograft material is not so limited, allograft is also disadvantageous because of the risk of disease transmission and immune reactions. Furthermore, allogenic bone does not have the osteogenic potential of autogenous bone and therefore will incorporate more slowly and less extensively.
These disadvantages have led to the investigation of bioactive substances that regulate the complex cascade of cellular events of bone repair. Such substances include bone morphogenetic proteins, for use as alternative or adjunctive graft materials. Bone morphogenetic proteins (BMPs), a class of osteoinductive factors from bone matrix, are capable of inducing bone formation when implanted in a fracture or surgical bone site. Recombinantly produced human bone morphogenetic protein-2 (rhBMP-2) has been demonstrated in several animal models to be effective in regenerating bone in skeletal defects. The use of such proteins has led to a need for appropriate carriers and fusion device designs.
In response to the needs still left unresolved by the prior devices, the present invention contemplates a hollow threaded interbody fusion device configured to restore the normal angular relation between adjacent vertebrae. In particular, the device includes an elongated body, tapered along substantially its entire length, defining a hollow interior and having an outer diameter greater than the size of the space between the adjacent vertebrae. The body includes an outer surface with opposite tapered cylindrical portions and a pair of opposite flat tapered side surfaces between the cylindrical portions. Thus, at an end view, the fusion device gives the appearance of a cylindrical body in which the sides of the body have been truncated along a chord of the body""s outer diameter. The cylindrical portions are threaded for controlled insertion and engagement into the end plates of the adjacent vertebrae.
In another aspect of the invention, the outer surface is tapered along its length at an angle corresponding, in one embodiment, to the normal lordotic curvature of lower lumbar vertebrae. The outer surface is also provided with a number of vascularization openings defined in the flat side surfaces, and a pair of elongated opposite bone ingrowth slots defined in the cylindrical portions. The bone ingrowth slots have a transverse width that is preferably about half of the effective width of the cylindrical portions within which the slots are defined.
In another embodiment, the interbody fusion device retains the same tapered configuration of the above embodiment, along with the truncated side walls and interrupted external threads. However, in this embodiment, the implant is not hollow but is instead solid. Bone ingrowth is achieved by forming the solid tapered implant of a porous high strength material that permits bone ingrowth into interconnected pores while retaining sufficient material for structural stability in situ. In one preferred embodiment, the material is a porous tantalum composite.
In another aspect of this invention, a hollow interbody fusion device is provided with an osteogenic material to optimize fusion. The osteogenic material comprises an osteoinductive protein in a suitable carrier.
In still another embodiment, the interbody fusion device is solid instead of hollow and is composed of a porous high strength material that permits bone ingrowth into interconnected pores. In one preferred embodiment, the material is coated with an osteoinductive material.
In another aspect a cap is provided which securely blocks the opening in a fusion device to prevent expulsion of an osteogenic material from within the device. The cap includes an occlusion body for blocking the opening and an elongated anchor for securing the occlusion body within the opening. In some embodiments the anchor includes a lip which is engageable to openings in the body wall.
In still another embodiment a tool is provided for manipulating caps for interbody fusion devices. In one embodiment the tool includes a pair of prongs each having facing engagement surfaces for engaging the fusion device, and a shaft slidably disposed between the prongs. The shaft has a cap-engaging tip for engaging a tool hole in the cap. The prongs include a pair of releasing members on each of the facing engagement surfaces. The releasing members have a height and a width for being insertable into apertures in a body wall in the fusion device to disengage elongate anchors of the cap from the apertures.